This invention concerns a crystallizer, or inner portion, of a mold for the continuous curved casting of thin slabs. To be more exact, this invention concerns a crystallizer which contains in its upper portion at a position in the center of the inner side of its longer sidewalls an enlargement hollow for introduction of the discharge nozzle of tundishes.
This enlargement hollow is necessary for introduction of the discharge nozzle since the width of the space for passage of the molten metal varies from 25 to 70 mm.; this enlargement hollow is disclosed in U.S. Pat. No. 2,564,723, which teaches the symmetrical enlargement of a straight, vertical crystallizer and is therefore not suitable for curved crystallizers.
SU-A-143.215 teaches that this enlargement should be shaped with a substantially cylindrical development, and this document too concerns a straight, vertical crystallizer.
EP-A-0.230.886 is associated with a straight, vertical crystallizer and discloses a symmetrical enlargement with inclined, vertical walls having a substantially rectangular development.
DE-A-3.501.422, EP-A-0.149.734 and DE-A-3.907.351 too disclose a straight, vertical crystallizer with a symmetrical enlargement.
EP-A-0.300.953 discloses a crystallizer of which the upper portion in which the discharge nozzle of the tundish is inserted is straight and vertical, whereas the lower portion is curved; the upper portion of the crystallizer may also include an arcuate part, which can be on one long side or the other, but the document does not give any geometric defining parameter; it does not in any way disclose the case of a wholly curved crystallizer but does provide for a change of direction of the skin of the thin slab being formed. This change of direction takes place where the straight, vertical portion is joined to the lower curved portion, and creates problems for the skin being formed owing to separations, interruptions in the surface, localized melting and reduced extraction speeds.
In the technology of the state of the art, the steel during continuous casting with partly curved molds of a known type undergoes dynamic effects due to the extraction movement and to the effects of shrinkage caused by cooling.
EP-A-0.276.418 and JP-A-58-97466 disclose lateral closure elements to determine the length of the casting chamber; the former document concerns straight, vertical crystallizers, while the latter concerns curved crystallizers.
Next, it should be borne in mind that it has been deemed hitherto that an enlargement having a lenticular shape without special limitations of a geometric type would be enough for the purpose.
Since in actual practice in enlargements having a lenticular shape, that is to say, in enlargements shaped with a curve having a radius of a given value there remains a concentration of tensions, caused by the geometric configuration, in the skin forming in contact with the zone uniting the lenticular shape to the surrounding surfaces, DE-A-3.907.351 teaches the inclusion of connecting curves, which, however, are not quantified dimensionally and are therefore contained in the state of the art, which teaches that a union should be created and that a sharp edge should not be left.
Moreover, according to the teachings of the state of the art relating, in particular, to partly curved molds there remains a tendency for the removal and/or tearing of the skin during the downward movement.
In fact, the skin tends to follow a line at a tangent to its own trajectory and the present applicants have found that if this line at a tangent forms, together with the surface of the crystallizer, an angle greater than a given value, separation may occur.
Such separation takes place when the skin is consolidated enough and the ferrostatic pressure is not high enough. In such cases a breakage of the skin by tearing may take place directly.